This invention relates generally to indexing systems of machines such as lathes and other machine tools. More particularly, the invention relates to an indexing system which is capable of producing a powerful mechanical holding action by which an indexed structure in determined index positions can be held positively against heavy loads, and which, moreover, has a locking mechanism functioning positively and accurately with respect to even an indexed shifting structure which moves at high speed.
Indexing systems and devices are widely used in machining operations. If the time required for carrying out the indexing of an indexing device is long, the total machining time will accordingly be lengthened and thus the productivity will be low. For this reason, there has been a great demand for high-speed indexing.
As one example, the indexing of the turret of a numerically-controlled (NC) lathe will be considered. Heretofore, the most common indexing process includes, along with use of a precision coupling, the sequential steps of (a) raising of the turret head to disengage a clamp of a coupling, (b) rotation of a turret indexing mechanism to carry out indexing of the turret head, and (c) retraction and clamping of the turret head in response to an indexing completion signal of the turret head. This indexing, however, requires a long time period of 1 to 3 seconds, so that the non-machining or non-productive time period is long in comparison to the total machining time, and thus this type of indexnig proves to be inefficient.
Furthermore, the above operational steps (a), (b), and (c) use hydraulic pressure and require two hydraulic circuits respectively for raising and retracting the turret head and for turret indexing. Accordingly, the system has been accompanied by the problems of high costs and complicated timing control.
Accordingly, an indexing system in which a geneva mechanism is utilized has been in use as a system capable of conforming to high-speed indexing. In this system, indexing movement is obtained by causing a pin of a crank functioning as a driving wheel to mesh intermittently with each of a plurality of radial slots provided in a driven wheel and is transmitted to the turret head. This system is provided further with a locking mechanism in which, after each indexing movement has been accomplished, a locking bolt is inserted and fitted by spring force into a bolt hole in a locking plate connected coaxially to this turret head thereby to hold and fix mechanically the indexed position, and, during the indexing movement, the locking bolt is extracted from the bolt hole overcoming the above mentioned spring force.
Since this locking mechanism depends solely on spring force acting on the locking bolt for locking, when a large load is exerted on the turret head as when heavy machining is being carried out, a reactive or counter force due to this large load acts to overcome at least partially the above mentioned spring force. Consequentially, the locking bolt is forced out of the bolt hole in the locking plate, whereby the turret head deviates from its index position.
This undesirable extraction movement of the locking bolt can be prevented by increasing the spring force (or spring constant), but this necessitates the use of a large spring. Furthermore, interrelatedly with the geneva mechanism, the cam mechanism for actuating the locking bolt in its insertion and extraction actions also becomes large, whereby it becomes difficult to make the indexing system compact, and, at the same time, the moving parts cannot respond quickly to high-speed motion. Accordingly, there is a limit to increasing the spring force.